The present invention relates generally to an apparatus for controlling a hydraulically-operated continuously variable transmission of belt-and-pulley type, particularly to a hydraulic control system or device including directional and flow control spool-valve units for controlling the shifting operation of the transmission. More particularly, the invention is concerned with improvements in operating response of the spool valves during their duty cycling flow control operations, and concerned with technologies for widening the range of control of the fluid flow in relation to a variation in the duty cycle.
A continuously variable transmission of belt-and-pulley type is known, which comprises an input shaft, an output shaft disposed parallel to the input shaft, a first variable-diameter pulley including a pair of opposed rotary members mounted on the input shaft, a second variable-diameter pulley including another pair of opposed rotary members mounted on the output shaft, and a transmission belt connecting the first and second variable-diameter pulleys. The speed ratio of the transmission is varied by changing the effective diameters of the variable-diameter pulleys. The effective diamter is changed by controlling the supply and discharge flows of a pressurized fluid into and from a hydraulic cylinder which is adapted to change the diameter of the pulley. An example of a hydraulic control system for a continuously variable transmission of belt-and-pulley type is disclosed in Japanese Patent Application 58-31298 laid open Sept. 10, 1984 under publication 59-159456 filed in the name of the assignee of the present application. In the case where such a transmission is used for an automotive vehicle, the speed of the engine can be optimally controlled by continuously changing the speed ratio of the transmission in response to the running conditions of the vehicle, so that the maximum fuel economy is obtained.
In such a hydraulic control system or device for the above-indicated transmission, there is employed a shift-speed control valve unit comprising (a) a valve body having a cylinder bore, (b) a spool which has a shaft portion, and plural spool lands larger in diameter than the shaft portion and spaced from each other along the axis of the shaft portion, and which is slidably received in the cylinder bore in the valve body, and (c) a spool actuator for operating the spool to selectively position the spool between its two axial positions. Upon axial movements of the spool, its spool lands selectively open and close the ports which communicate with conduits to supply and discharge the fluid to and from the previously indicated hydraulic cylinder to change the effective diameter of the variable-diameter pulley of the transmission. With the opening and closing of the ports, the fluid supply and discharge flows to and from the hydraulic cylinder are controlled, i.e., either permitted or restricted, and the rate of variation in the speed ratio of the transmission (i.e., shifting speed of the transmission) is controlled. In this valve unit, a pair of ports are provided in the discharge conduit, one communicating with the upstream side of the conduit and the other communicating with the downstream side. In addition, another port is provided in communication with a restrictor passage for restricting the fluid flow from the hydraulic cylinder. Thus, a total of three ports are provided in association with the discharge of the fluid from the cylinder. Generally, the above indicator pair of ports are brought into communication with each other to permit the discharge flow, and the restrictor port is put into communication with one of said pair of ports to restrict the fluid flow. In such an arrangement having a total of three ports, the valve spool should have a relatively large axial dimension (length) and is therefore constructed with a relatively large mass. Therefore, the operating response of the valve unit is not sufficiently high. This means a comparatively narrow range of duty-cycling control of the fluid flow in relation to a variation in the duty cycle of the valve unit. Thus, the known hydraulic control arrangement for a continuously variable transmission suffers considerable difficulty in assuring smooth and high-precision control of the rate of variation in the speed ratio of the transmission, i.e., shifting speed of the transmission.
The known hydraulic control device also includes a shift-direction switching valve unit for changing the direction of flow of the pressurized fluid to supply or discharge the fluid to or from the hydraulic cylinder for changing the direction of shifting of the transmission. This valve unit is similar in construction to the above-described shift-speed control valve unit, and has the similar inconvenience in terms of the operating response.